Sorry, just getting back to this after a busy work week! Apologies for delay.
This is kind of an early time to molt, but if you are seeing feathers around, more so than usual, that would explain not laying. Also they may stop laying from any kind of stress, such as a lot heat. But it's the staying in the coop that's weird. They do get quiet and low energy sometimes when molting. So there are no indications other than that that she is sick - is she fluffed up? droopy? Those are bad signs. Also check her poop and there's links to a great poop analysis page if you search here. Check her crop (yes, you'll need to do that by hand so you'll have to make her let you) to make sure it isn't really hard, as in impacted, or really squishy. But you said she is eating and drinking fine....
If you've had her 2 years you would have seen her go through a molt before and it sounds like she didn't act that way. I'm not much of an expert; I just had a hen with internal laying so I know about that. if you can watch and record any other signs, maybe someone else can provide more help. A week later, it should be more clear if she is in a molt and that might help you determine. You can also give her electrolytes if you like; I understand they perk up with that. I will tend to put ACV in their water and give yogurt with chopped garlic to support their immune system, and that can be good during a molt or when they seem out of sorts. I am more of the organic-only type and other people can tell you a lot more about meds.
